Recently demand for precise electric and electronic apparatuses which are small-sized and portable, such as audio tape recorder, camera built-up video tape recorder, personal computer and portable phone has been increasing more and more. As a result of such an increased demand, there has come to be required so-called secondary battery which is light and rechargeable, provides power source for driving those apparatuses and has high energy density. New secondary batteries of nickel-metal hydride type and lithium type have been commercialized by turns in addition to conventional lead storage batteries and nickel-cadmium secondary batteries.
A factor which greatly influences performance of those batteries is an active material for a battery (hereinafter referred to simply as "active material") among materials for positive and negative electrodes of a battery. Examples of the active material for positive electrode are, for instance, manganese dioxide (MnO.sub.2), nickel hydroxide (Ni(OH).sub.2), lithium cobalt dioxide (LiCoO.sub.2), lithium nickel dioxide (LiNiO.sub.2), vanadium pentoxide (V.sub.2 O.sub.5), niobium pentoxide (Nb.sub.2 O.sub.5), lithium manganese dioxide (LiMnO.sub.2) and the like, and examples of the active material for negative electrode are, for instance, cadmium hydroxide (Cd(OH).sub.2), hydrogen storage alloy (M--H), carbonaceous materials (natural graphite, artificial graphite, coke, etc.) and the like.
When producing a battery, those active materials are bound to electroconductive carbonaceous materials, for example, carbon blacks such as acetylene black and Ketjen black and graphites to produce an electrode. In such a case, fluorine-containing resins being excellent in chemical resistance and heat resistance and having binding property have been used widely as a suitable binder.
Among the above-mentioned fluorine-containing resins, as an example of use of polytetrafluoroethylene as a binder, for instance, JP-A-63-236258 discloses use of a dispersion of high molecular weight polytetrafluoroethylene (PTFE)(homopolymer) for binding MnO.sub.2 which is the active material for positive electrode of lithium primary battery with an electroconductive carbonaceous material such as acetylene black or graphite. JP-B-6-10980 discloses use of a dispersion of high molecular weight PTFE (homopolymer) for binding manganese oxide which is the active material for positive electrode of air-zinc battery with carbon black and activated carbon which are electroconductive carbonaceous materials.
On the other hand, there is known a disclosure of use of poly(vinylidene fluoride) (PVDF) as a binder. For example, JP-A-6-44964 discloses an electrode of a nickel-metal hydride secondary battery which is produced by mixing hydrogen storage alloy and carbonyl-nickel powder which are the active materials with a PVDF solution and molding the mixture into a sheet.
As a property for evaluating performance of such secondary batteries, there is used a cycle life. In secondary batteries employing nickel compound as the active material for positive electrode, particularly a nickel-metal hydride secondary battery, at the time of over-charging or rapid charging by large current, oxygen gas is generated from the positive electrode and an internal pressure of the battery increases. This is the main cause for shortening the cycle life. Namely, when the internal pressure of the battery increases over a certain level and a safety valve mounted is actuated, an aqueous solution of electrolyte and oxygen gas are released outside the battery to lower discharge capacity and cause leakage of the solution, which results in a problem of shortening of battery life.
To solve the mentioned problem, there is a method to diffuse the generated gas quickly and accelerate a reaction for converting the generated gas to water, an oxide or a hydroxide. As means therefor, there have been already practically employed a method wherein a balance of capacity of the positive electrode and negative electrode is adjusted to increase the capacity of the negative electrode and a method wherein a carbon supporting reduction catalyst is added to the negative electrode. Another method is such that since the reaction for diffusing the generated gas quickly to convert it to water, oxide or hydroxide occurs on an interface of three-phase (gas phase-liquid phase-solid phase) within the electrodes, such an interface is allowed to be present finely, closely and uniformly inside the electrodes (particularly negative electrode) or on the surface thereof.
In the nickel-metal hydride secondary battery, a fluorine-containing resin is used because it easily forms the interface of three-phase and has a low surface energy and excellent water repellency, and thus not only a function of binding property but also a function of water repellency are utilized.
Also a reaction mechanism of the fuel cell is such that the reaction positively occurs on the interface of three-phase of fuel gas, electrolyte and catalyst, and the fluorine-containing resin used therefor plays the both roles of binding and water repellency.
Among the fluorine-containing resins, particularly PTFE which is prepared by emulsion polymerization has a characteristic of being easily fibrillated by a small compressive shear force unless previously heat-treated at a temperature of not less than its melting point. Therefore, if PTFE is mixed with other materials, fibrils are easily produced and exhibits a function of binding them, namely binding property. The reason why the fluorine-containing resin, particularly PTFE is used as the binder for the electrode of the battery lies in such a binding property and very excellent chemical resistance and heat resistance, and thus used in various primary and secondary baterries (for example, air-zinc primary battery, alkali-manganese primary battery, lithium primary battery, nickel-cadmium secondary battery, nickel-metal hydride secondary battery, lithium ion secondary battery and the like). However, in the nickel-cadmium secondary battery, nickel-metal hydride secondary battery or fuel cell which requires water repellency as well as binding property, a small amount of PTFE suffices for fibrillation, but in order to endow the electrode with water repellency closely and uniformly, it is necessary to use PTFE in an amount of not less than required amount for binding. When water repellency is enhanced by increasing an amount of PTFE, some difficulties arise usually. For example, since fibrillation is easy to occur, it is substantially difficult to mix PTFE uniformly with other liquid or powder materials and there is a case where water repellency cannot be always uniformly given to the electrode. Further, the active material must be decreased in an amount corresponding to the increased amount of PTFE. Either case result in impairing of electrode characteristics or battery performance. An average molecular weight of the PTFE is over 1,300,000 up to 10,000,000. Any of PTFE homopolymer and modified PTFE may be used if they are fibrillated.
Under the background mentioned above, there is proposed use of relatively low molecular weight PTFE (homopolymer) to prevent the fibrillation. For example, it is recommended in JP-A-5-242908 to use a fluorine-containing resin powder having an average molecular weight of not more than 3,000,000, concretely a low molecular weight PTFE (homopolymer) and tetrafluoroethylene-hexafluoropropylene copolymer (hereinafter referred to as "FEP"). However, the fluorine-containing resin disclosed in the above-mentioned patent publication is in the form of powder, and it is usually difficult to mix the resin to an active material of batteries and coat it to the electrode uniformly. Furthermore, in case of the PTFE (homopolymer) having the average molecular weight of around 3,000,000, the fibrillation is generated to a certain extent, and such an average molecular weight is insufficient for exhibiting an effect which the above patent publication aimed at.
An object of the present invention is to provide a water-repelling agent for batteries, which can be obtained without using a fluorine-containing resin powder as it is, causes no fibrillation even if mixed with the electrode materials, can enhance chemical resistance, heat resistance and particularly water repellency more effectively and is excellent in workability, and also to provide a battery which is produced by using the water-repelling agent and is excellent in cycle life and internal pressure characteristics.